rtlwifi: rtl8192cu: Fix sparse non static symbol warning

Fixes the following sparse warning:

drivers/net/wireless/rtlwifi/rtl8192cu/hw.c:1595:6: warning:
 symbol 'usb_cmd_send_packet' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>